A Herschel view of the far-infrared properties of submillimetre galaxies^⋆,⋆⋆

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Giessenbachstraße 1, 85741 Garching, Germany
e-mail: magnelli@mpe.mpg.de
² INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
³ Argelander Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
⁴ Herschel Science Centre, ESAC, Villanueva de la Cañada, 28691 Madrid, Spain
⁵ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁶ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
⁷ Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d’Astrophysique, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
⁸ Instituto de Astrofísica de Canarias (IAC), C/vía Láctea S/N, 38200 La Laguna, Spain
⁹ Departamento de Astrofísica, Universidad de La Laguna, Spain
¹⁰ California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125, USA
¹¹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹² Dipartimento di Astronomia, Università di Bologna, Via Ranzani 1, 40127 Bologna, Italy
¹³ Department of Physics & Astronomy, University of California, Irvine, CA 92697, USA
¹⁴ Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
¹⁵ Universität Wien, Institut für Astronomie, Türkenschanzstraße 17, 1180 Wien, Österreich
¹⁶ SUPA (Scottish University Physics Alliance), Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
¹⁷ Astronomy Centre, Dept. of Physics & Astronomy, University of Sussex, Brighton BN1 9QH, UK
¹⁸ Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138, USA
¹⁹ UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
²⁰ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
²¹ Dipartimento di Astronomia, Universita di Padova, Vicolo dell’Osservatorio 3, 35122, Italy
²² Max-Planck-Institut für Plasmaphysik, Boltzmannstraße 2, 85748 Garching, Germany
²³ Excellence Cluster Universe, TUM, Boltzmannstraße 2, 85748 Garching, Germany
²⁴ Department of Physics & Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada

Received: 21 October 2011
Accepted: 30 January 2012

Abstract

We study a sample of 61submillimetre galaxies (SMGs) selected from ground-based surveys, with known spectroscopic redshifts and observed with the Herschel Space Observatory as part of the PACS Evolutionary Probe (PEP) and the Herschel Multi-tiered Extragalactic Survey (HerMES) guaranteed time key programmes. Our study makes use of the broad far-infrared and submillimetre wavelength coverage (100−600   μm) only made possible by the combination of observations from the PACS and SPIRE instruments aboard the Herschel Space Observatory. Using a power-law temperature distribution model to derive infrared luminosities and dust temperatures, we measure a dust emissivity spectral index for SMGs of β = 2.0 ± 0.2. Our results unambiguously unveil the diversity of the SMG population. Some SMGs exhibit extreme infrared luminosities of s10${}^{\mathrm{13}}\mathit{L}_{\mathrm{\odot }}$ and relatively warm dust components, while others are fainter (a few times 10¹²   L_⊙) and are biased towards cold dust temperatures. Although at zs2 classical SMGs (>5 mJy at 850   μm) have large infrared luminosities (s10¹³   L_⊙), objects only selected on their submm flux densities (without any redshift informations) probe a large range in dust temperatures and infrared luminosities. The extreme infrared luminosities of some SMGs (L_IR ≳ 10^12.7   L_⊙, 26/61 systems) imply star formation rates (SFRs) of  >500 M_⊙ yr^-1 (assuming a Chabrier IMF and no dominant AGN contribution to the FIR luminosity). Such high SFRs are difficult to reconcile with a secular mode of star formation, and may instead correspond to a merger-driven stage in the evolution of these galaxies. Another observational argument in favour of this scenario is the presence of dust temperatures warmer than that of SMGs of lower luminosities (s40 K as opposed to s25 K), consistent with observations of local ultra-luminous infrared galaxies triggered by major mergers and with results from hydrodynamic simulations of major mergers combined with radiative transfer calculations. Moreover, we find that luminous SMGs are systematically offset from normal star-forming galaxies in the stellar mass-SFR plane, suggesting that they are undergoing starburst events with short duty cycles, compatible with the major merger scenario. On the other hand, a significant fraction of the low infrared luminosity SMGs have cold dust temperatures, are located close to the main sequence of star formation, and therefore might be evolving through a secular mode of star formation. However, the properties of this latter population, especially their dust temperature, should be treated with caution because at these luminosities SMGs are not a representative sample of the entire star-forming galaxy population.